into the hollow of the bottom rail; and as soon as the door begins opening, the roller sinks and turns at the same time, without obstructing the door from opening.

Fig. 2, shows how the roller is fixed in the centre of the opening of the door, so that when the door is shut, the roller will be in the centre of the bottom rail. I 3, is intended to show that the roller is fixed a little below the centre or top of the floor. Let the springs be made light, and screwed down, and then there will be found no difficulty in getting the roller in. A place may be made at D, to carry off any water that may fall in. It is obvious, however, that by the above arrangement no water can pass into the passage

or room.

From fig. 3 it will be seen, that

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In the above figure a, b, c, d, represents a drum-wheel or cylinder, moving on a horizontal axis, surrounded with a band of sponge, 12 3 4 5 6 7 8, and immersed in water, so that the surface of the water touches the lower end of the cylinder. Now then, if, as in fig. 2, in our last number, the water on the descending side b, be allowed to accumulate in the sponge at a, while on the ascend ing side D, the sponge at the water's edge shall, by any means not deranging the equilibrium, be so compressed, that it shall quit the water in a dry state; the accumulation of water

above its level at x, by the capillary attraction, will be a source of constant rotatory motion; and in the present case it will be found that the means of compressing the sponge may be best obtained by buoyancy instead of weight.

For this purpose, therefore, the band of sponge is supposed to be divided into eight or more equal parts, 1234, &c. each part being furnished with a float or buoyant vessel, f1, f 2, &c. rising and falling upon spindles s s s, &c. fixed in the periphery of the drum; these floats being of such dimensions, that when

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immersed in water, the buoyancy or pressure upwards of each, shall be sufficient to compress that portion of the sponge connected with it, so as to squeeze out any water it may have absorbed. These floats are further arranged by means of levers, 1 1 1, &c. and plates, ppp, &c. so that when the float, fNo. 1, becomes immersed in the water, its buoyant pressure upwards, acts not against the portion of the sponge, No. 1, immediately above it, but against No. 2, next in front of it; and so in like manner the buoyancy of f, No. 2 float, acts on the portion of sponge, No. 3, and f, No. 3 float, upon No.4 sponge.

Now, from this arrangement it follows that the portion of sponge No. 4, which is about to quit the water, is pressed upon by that float, which from acting vertically, is most efficient in squeezing the sponge dry, while that portion of the sponge, No. 1, on the point of entering the water, is not compressed at all from its corresponding float No. 8, not having yet reached the edge of the water. By these means, therefore, it will be seen that the sponge always rises in a dry state from the water, on the ascending side, while it approaches the water on the descending side in an uncompressed state, and open to the full action of absorption by the capillary attraction.

The great advantage of effecting this by the buoyancy of light vessels instead of by a burthen of weights, as in fig. 2, in our last number, is, that by a due arrangement of the dimensions and buoyancy of the floats im mersed, the whole machine may be made to float on the surface of the water, so as to take off all friction whatever from the centre of suspension. Thus, therefore, we have a cylindrical machine revolving on a single centre without friction, and having a collection of water in the sponge on the descending side, while the sponge on the ascending side is continually dry; and if this cylinder be six feet wide, and the sponge that surrounds it one foot thick, there will be a constant moving power of

thirty pounds on the descending side without any friction to counteract it. It has been already stated, that to perpetuate the motion of this ma chine, the means used to leave the sponge open on the descending side and press it dry on the ascending side, must be such as will not derange the equilibrium of the machine, when floating in water. As, therefore, in this case the effect is produced by the ascent of the buoyant floats b, to demonstrate the perpetuity of the motion, we must show that the ascent of the floats f No. 1 and f No. 3. will be equal in all corresponding situations on each side of the perpendicular; for the only circumstance that could derange the equilibrium on this system, would be that f No. 1, and ƒ No. 3, should not in all such corresponding situations approach the centre of motion equally; for it is evident that in the position of the floats described in the above figure, if ƒ No. 1 float did not approach the centre as much as f No. 3, the equilibrium would be destroyed, and the greater distance of f No. 1 from the centre than that of f No. 3, would create a resistance to the moving force caused by the accumulation of the water at x.

It will be found, however, that the floats ƒ No. 1, and ƒ No. 3, do retain equal distances from the centre in all corresponding situations, for the resistance to their approach to the centre by buoyancy is the elasticity of the sponge at the extremity of the respective levers; and as this elasticity is the same in all situations, while this centripetal force of the float f No. 1 is equal to that of the float ƒ No. 3, at equal distances from the perpendicular, the floats f No. 1 and fNo. 3, will in all corres ponding situations on either side of the perpendicular be at equal distances from the centre. It is true that the force by which these floats approach the centre of motion varies according to the obliquity of the spindles on which they work, it being greatest in the perpendicular position; but as the obliquity of these spindles is the same at all equal distances from the perpendicular, and as

the resistance to the ascent of the floatsi s equal in all cases, the centre of buoyancy will evidently describe a similar curve on each side of the perpendicular; and consequently the equilibrium will be preserved, so as to leave a constant moving force at x, equal to the whole accumulation of water in the sponge. Nor will this equilibrium be disturbed by any change of position in the floats not immersed in the water, since, being duly connected with the sponge by the levers and plates, they will evidently arrange themselves at equal distances from the centre, in all corresponding situations on either side.

It may be said, that the equilibrium of the band of sponge may be destroyed by its partial compression; and it must be admitted, that the centre of gravity of the part compressed, according to the construction above described, does approach the centre of motion nearer than the centre of gravity of the part not compressed. The whole weight of the sponge is, however, so inconsiderable, that this difference would scarcely produce any sensible effect; and if it did, a very slight alteration in the construction, by which the sponge should be compressed as much outwards as inwards, would retain the centre of gravity of the compressed part at the same distance from the centre of motion, as the centre of gravity of the part not com pressed.

THE PATENT LAWS.

Sir, I am truly pleased to see that you have, at length, taken up the subject of the patent laws; and I hope, most sincerely, that your efforts to obtain EMANCIPATION for mechanical genius, will be attended with the success they deserve. No one, I am satisfied, who will inform himself of the condition of those laws, can doubt the necessity for their immediate alteration. Can any thing be more palpably unjust and injurious, than to impose a tax on the most productive of all the powers of man-his faculty of invention? Can

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any thing be more cruel, than to have laws in operation, which oblige the indigent inventor to dispose of the fruits of his labour, for a mere nothing, and thereby add fresh accumulations to the already abundant hoards of the rich? What equity can there be in inflicting on one description of intellect, a state of comparative inactivity, (for such is, in fact, the effect produced by the patent laws,) while every other is perfectly free ?*

I will now proceed to another consideration. The sum received in

England for patents, does not exceed 12,000l. yearly; and not one penny of that sum falls into the public funds. On the contrary, every fraction of it is consumed by the cormorants in office, into whose gaping and capacious pockets, it silently drops. Is it not obvious, then, that the law has only benefited a few individuals, who are otherwise amply remunerated, to the grievous oppression of the many?

In order to shew how much humble genius sometimes suffers from these laws, I shall give an instance which came under my own observation.

About nine months since, a young man made a most useful invention. From want of means, he was obliged to sell it for 1891. 9s. Out of this he had to pay 1391. 9s. for letters patent; so that the paltry residue of 501. was all that he had to remune rate him for his trouble and ingenuity, In fact, he scarcely pocketeda shilling by the speculation, for it took nearly 501. to pay the expense of constructing the requisite model, while the purchaser, a wealthy man, has actually not only been reim

That the ancient Egyptians shoud, worship bulls, cats, and crocodiles cannot excite greater wonder in the mind of the traveller who beholds the remains of the spacious and magnificent receptacles erected for their carcasses, than that which posterity will experience, (while contemplating the mechanical discoveries of the present age,) when they think of the obstructions with which the legislature encumbered the energies of the mechanics by whom those discoveries were ef fected.

bursed his purchase money, but has cleared a considerable profit, although the letters patent have not been granted more than nine months. Nor is this a solitary instance; many more such might be adduced.

As some of the readers of your excellent magazine may not be aware of the great expense of letters patent, it may not be improper to state the price, separately, for each kingdom. The cost of a patent for England, in the usual form, when unopposed, exclusive of the specification, is 1051. for Scotland 751. for Ireland 1201. total, 3001. Therefore, the individual who wishes to obtain letters patent for the three kingdoms, with the cost of the specifications and lawyer's expenses, must pay between 3501. and 4001. and even then, after all the time, money, and labour, spent first in making the discovery, and secondly in obtaining the letters patent, should there happen to be ever so slight a mistake in the specification, it is all lost to the patentee. Sincerely hoping soon to see the emancipation of mechanical genius, I remain, Sir,

Your very obedient Servant, JAMES BARLOW FERNANdez. 4, Norfolk Street, Strand.

ENGLISH ROPEMAKING. One of the most remarkable examples of the advantage of substituting scientific mechanical combinations for ordinary manual operations, is displayed in the rope machinery of England. By this machinery the English now obtain ropes of equal strength, with one third less material and sometimes even less, according to the size of the rope.→→ Dupin's Geometry of the Arts."

AEROSTATION.

A correspondent, who has directed his attention to aerostation, and invented a method of increasing the buoyancy of hydrogen gas, so as to enable the balloon to be diminished to one-tenth of the present size required, is desirous of obtaining the ideas of our readers on the subject of a rotary motion, which will exert

a considerable power in the air. The great desideratum is, that the fans or paddles should open during half of their giration, and shut at the other half, with as much velocity and as little friction as possible.

FOOD FOR HORSES.

A practice is becoming general in Silesia of feeding horses with bread, made by taking equal quantities of oat and rye meal, mixing it with leaven or yeast, and adding one third of the quantity of boiled potatoes. To each horse is given 12lbs. per day, in rations of 4lbs. each. The bread is cut into small pieces, and mixed with a little moistened cut straw. It is stated that by this means there is a saving in feeding seven horses "of 49 bushels of oats in 24 days, while the horses perform, their common labour, and are much better in look, health, and disposition."

Another improvement in the feeding of horses, which has just been announced by the Rev. W. Evans, of Llande feilog, Carmarthenshire, con-, sists in the subsitution for hay and corn, of cut straw and potatoes, or straw, chaff, and pounded furze mixed, wetted with salt water. The process of preparing it is as follows: "Let a tub full of fresh water, with an egg in it, be impregnated with as much domestic salt as will cause the egg to rise and float on the surface, that being the criterion of its saltness, being equal to that of sea water. The provender being put into a wicker basket, and placed on the tub, pour the salted water upon it, in quantity sufficient to wet the whole mass, and when it shall have done filtering through it, give it to the horses. The salted water will not only moisten and sweeten the food, but also operate as a most ef ficient alterative to purify the blood, purge all gross humours, prevent the increase of worms, and all painful attacks from those troublesome vermin. Horses fed in this manner will work well, and will be fit for all sorts of work. A man cuts with one knife machine, in four hours, enough of wheaten straw to last nine horses for twenty-four hours."

THE LINE OF CHORDS.

The chief applications of the line of lines being thus explained, it is hoped, with sufficient precision, we must pass to the trigonometrical lines; and, to take these in order, we will begin with the line of chords. This pair of lines, like all the other double lines, is of the same length with this line, and makes the same angle at the centre. The advantage of which disposition is, that a space taken upon any of the double lines, may be instantly reduced to numbers by applying it laterally to the line of lines. For explaining the construction of the trigonometrical lines, I shall make use of the usual elementary figure, which shews the description of all, whether plane or sectoral; but, in treating of their uses, I shall confine myself to those only whose applications are likely to occur in the practice of the generality of readers. H

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For, I conceive, few, comparatively, who resort to the Mechanics' Mag.

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for the sake of instruction to be derived from its pages, have any occa